Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: DS2220 ).

Acknowledgements

We thank the Department of Science and Technology (DST), India, for the CCD facility at the Indian Institute of Science (IISc), Bangalore. CSAK thanks the University Grants Commission for the award of a Dr D. S. Kothari post-doctoral fellowship. GM thanks the Government of India for the award of a National Research Professorship and acknowledges the current research support from the Eli Lilly and Jubilant-Bhartia Foundations.

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